Bipolar chopping for 1/f noise and offset reduction in magnetic field sensors

ABSTRACT

A chopping technique, and associated structure, is implemented to cancel the magnetic 1/f noise contribution in a Tunneling Magnetoresistance (TMR) field sensor. The TMR field sensor comprises a first bridge circuit including multiple TMR elements to sense a magnetic field and a second circuit to apply a bipolar current pulse adjacent to each TMR element. The current lines are serially or sequentially connected to a current source to receive the bipolar current pulse. The field sensor has an output comprising a high output and a low output in response to the bipolar pulse. This asymmetric response allows a chopping technique for 1/f noise reduction in the field sensor.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims the benefit of U.S. Provisional PatentApplication No. 61/993,598, filed May 15, 2014, which application ishereby incorporated herein by reference in its entirety.

BACKGROUND

A. Technical Field

The present invention relates generally to the field of magnetic fieldsensors and more particularly to method of noise and offset reduction inmagnetic field sensors.

B. Background of the Invention

Magnetic field sensors have been commonly used in various electronicdevices, such as computers, laptops, media players, smart phones, etc.There are several techniques/devices that can be used for detecting amagnetic field. Tunneling Magnetoresistance (TMR) is a promisingmagnetic sensing technology for handset applications due to itsadvantages in sensitivity, power, and process cost compared with othermagnetic sensors. Another closely related technology in magnetic fieldsensing is Giant Magnetoresistance (GMR).

A TMR element is composed of two ferromagnetic layers separated by anon-magnetic, insulating tunnel barrier. One layer has a magnetizationdirection that is “free” to rotate in a magnetic field. The other layerhas a “fixed,” reference magnetization that does not rotate when in amagnetic field of moderate to low strength that is of sensing interest.If the magnetization directions of the two layers are parallel to eachother, the electrical resistance of the tunnel barrier is low.Conversely, when the magnetization directions are anti-parallel, theresistance is high. A magnetic field sensor based on TMR thereforeconverts magnetic field into electrical signal by a change in electricalresistance due to the changing angle of the magnetic free layer relativeto the fixed layer in response to the field.

In this type of sensor, a magnetic 1/f noise is caused by fluctuationsin the magnetic orientation of regions in one or both of the layers.These fluctuations create changes in resistance at different time scalesthat appear as noise in the target signal and cannot be simply filteredout. A 1/f noise (also called pink noise) is a signal or fluctuationprocess with a frequency spectrum density (energy or power per Hz)inversely proportional to the frequency of the signal. In addition, azero-field offset can also be created by differences in the magneticorientation of regions in either magnetic film, which may be dependenton temperature or stress in the sensor.

Therefore, it would be desirable to have a system, device and method toeffectively reduce 1/f noise and offset in TMR magnetic field sensors.

SUMMARY OF THE INVENTION

Certain embodiments of the invention provide for systems, devices andmethods using a unique, bipolar chopping technique to cancel themagnetic 1/f noise and offset contribution within TMR sensors. Aspreviously described, magnetic 1/f magnetic noise exists in TMR magneticfield sensors and can reduce the performance and sensitivity of thesesensors. At low frequencies (<100 Hz) used for compassing and motionsensing applications, the 1/f magnetic noise contribution becomesdominant. This noise manifests as a low frequency meander in the zerofield offset of a TMR sensor bridge, thus reducing the accuracy.

According to various embodiments of the invention, chopping technique,and associated structure, is implemented to cancel the magnetic 1/fnoise contribution and offset in a TMR field sensor. The TMR fieldsensor comprises a first bridge circuit including a plurality of TMRelements configured to sense a magnetic field. Each TMR elementcomprises a first and a second ferromagnetic layers separated by anon-magnetic, insulating tunnel barrier. The TMR field sensor furthercomprises a second circuit configured to apply a bipolar current pulseadjacent to each of the TMR elements.

In certain embodiments, the second circuit comprises a plurality ofbuilt-in current lines placed adjacent to the second (fixed)ferromagnetic layer of each TMR element of the first bridge circuit. Thecurrent lines are serially or sequentially connected to the fieldsource, and the bipolar current pulse is applied to all of the currentlines. When the bipolar current pulse is applied, a magnetic field pulseis generated on the first (free) ferromagnetic layer. Depending on thepolarity of the applied current pulse, the generated magnetic fieldswitches the first free ferromagnetic layer into a first or secondalignment.

When the bipolar current pulse is applied adjacent to each of the TMRelement, the free layer orientation switches to align with the currentpulse polarity. After the pulse completes, the sensor output is measuredfor that polarity of free layer alignment. The measured sensor outputsignal measured at each of the free layer polarities does not changesign, but the magnetic 1/f noise contribution and offset does. Thisasymmetric response allows a chopping technique where the magnetic fieldis applied in a first direction at a first period, then to a seconddirection at a second period with a repetitive rate of at least twotimes the desired output data rate. The sensor output signals at bothperiods are combined so the magnetic field sensing signal isconstructively added, but the 1/f noise and offset contributions arecancelled. In one embodiment, the applied magnetic fields in said firstdirection and said second direction align completely with the freeferromagnetic layer. In another embodiment, the applied magnetic fieldsin said first direction and said second direction align in an off angleorientation with the free ferromagnetic layer for a desired (or better)switching distributions.

All aspects of this invention will directly apply to devices based onGMR as well. The inventions disclosed here also apply to any magneticsensing technology that utilizes soft-magnetic films for sensingmagnetic fields, for Example AMR, Fluxgate, Hall with a fluxconcentrator. For simplicity and clarity, the invention will bedescribed in this document using TMR technology as an example.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to exemplary embodiments of the present inventionthat are illustrated in the accompanying figures. Those figures areintended to be illustrative, rather than limiting. Although the presentinvention is generally described in the context of those embodiments, itis not intended by so doing to limit the scope of the present inventionto the particular features of the embodiments depicted and described.

FIG. 1 is an exemplary structure overview of a TMR magnetic field sensoraccording to various embodiments of the invention.

FIG. 2 is a cross-section view of a single TMR element according tovarious embodiments of the invention.

FIG. 3 is an exemplary diagram of two different chopping statesaccording to various embodiments of the invention.

FIG. 4 is an exemplary diagram of a bridge circuit for Z-axis accordingto various embodiments of the invention.

FIG. 5 is an exemplary control current signal according to variousembodiments of the invention.

FIG. 6 is an exemplary sensor output signal according to variousembodiments of the invention.

FIG. 7 is a diagram of validation measurement results according tovarious embodiments of the invention.

FIG. 8 is an exemplary diagram of multiple muxed (multiplexed) H bridgesfor sequential reset and control logic according to various embodimentsof the invention.

FIG. 9 is an exemplary diagram of read chain according to variousembodiments of the invention.

One skilled in the art will recognize that various implementations andembodiments of the invention may be practiced in accordance with thespecification. All of these implementations and embodiments are intendedto be included within the scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for the purpose of explanation, specificdetails are set forth in order to provide an understanding of thepresent invention. The present invention may, however, be practicedwithout some or all of these details. The embodiments of the presentinvention described below may be incorporated into a number of differentelectrical components, circuits, devices, and systems. Structures anddevices shown in block diagram are illustrative of exemplary embodimentsof the present invention and are not to be used as a pretext by which toobscure broad teachings of the present invention. Connections betweencomponents within the figures are not intended to be limited to directconnections. Rather, connections between components may be modified,re-formatted, or otherwise changed by intermediary components.

When the specification makes reference to “one embodiment” or to “anembodiment”, it is intended to mean that a particular feature,structure, characteristic, or function described in connection with theembodiment being discussed is included in at least one contemplatedembodiment of the present invention. Thus, the appearance of the phrase,“in one embodiment,” in different places in the specification does notconstitute a plurality of references to a single embodiment of thepresent invention.

Various embodiments of the invention are used for 1/f noise reductionand offset reduction in TMR magnetic field sensors used in variouselectronic devices, such as computers, laptops, media players, smartphones, wearable electronics, etc. The TMR magnetic field sensors, andthe single TMR element therein, may be integrated on a single componentor contain discrete components. Furthermore, embodiments of theinvention are applicable to a diverse set of techniques and methods.

FIG. 1 shows a schematic diagram of a TMR magnetic field sensor 100according to various embodiments of the invention. The magnetic fieldsensor 100 comprises a first bridge circuit 200 powered by a voltagesource 300, a second circuit 400 powered by an optional stabilizationfield source 500. In one embodiment, the field source 500 is a currentsource. The first bridge circuit 200 comprises a plurality of TMRelements 210. The bridge circuit 200 may be a half bridge circuit, afull bridge circuit, or any combinations thereof. In one embodiment, thebridge circuit 200 is a bridge circuit having two circuit branches withbridge output signal 260 between the two branches at some intermediatepoint along the branches.

FIG. 2 illustrates a cross-section view of a single TMR element 210according to various embodiments of the invention. The TMR sensor 210 iscomposed of a first ferromagnetic layer 212 and a second ferromagneticlayer 214 separated by a non-magnetic, insulating tunnel barrier 216. Inone embodiment, the first layer 212 has a magnetization direction 232that is free to rotate in a magnetic field. The second layer 214 has afixed reference magnetization direction 234 that does not rotate when ina magnetic field. If the magnetization directions of the two layers areparallel to each other, the electrical resistance of the tunnel barrier216 is low. Conversely, when the magnetization directions areantiparallel, the resistance is high. The TMR element therefore convertsmagnetic field into electrical signal by the changing the electricalresistance due to the changing angle of magnetic free layer relative tothe fixed layer in response to the field. The ferromagnetic layers 212and 214 may be formed from any suitable ferromagnetic material, such asNi, Fe, Co or their alloys. The insulating tunnel barrier 216 may becomposed of insulator materials such as AlOx, MgOx, ZrOx, TiOx, HfOx, orany combinations thereof.

In one embodiment, the first ferromagnetic layer 212 is connected to afirst conductive line 224 by a first contact 222, and the secondferromagnetic layer 214 is connected to a second conductive line 228 bya second contact 226, which may contact from above as well as below thesecond ferromagnetic layer.

In one embodiment, the second circuit 400 comprises a plurality ofbuilt-in current lines 410 placed adjacent to the second ferromagneticlayer 214 of each TMR element 210. The current lines 410 are connectedsuch that a current pulse is applied to the current line for each TMRelement 210. The connection of the current lines 410 may be sequential,serial or time multiplexed according to various embodiments. In anotherembodiment, the current line may also be placed adjacent to the firstferromagnetic layer 212, or placed adjacent to both the first and secondferromagnetic layers of each TMR element 210. The first ferromagneticlayer 212 is patterned into a shape that has a long and a short axis. Inzero magnetic field, the magnetization direction of the firstferromagnetic layer 212 lies along the long axis of the element and canbe directed either of the two directions along this axis. By applying acontrol current signal to the current line 410, an induced magneticfield is generated in an ambient area surrounding the current line.Since the first layer 212 has a magnetization direction 232 free torotate and switch, the magnetization direction 232 will switch to alongthe direction projected on its axis by the induced magnetic field. As anexemplary illustration in FIG. 2, when the current in the current line410 has a direction pointing outward, the magnetization direction 232points leftward, which is has a component that is negatively aligned tothe reference magnetization direction 234, and will switch the freelayer to the left; when the current in the current line 410 has adirection pointing inward, the magnetization direction 232 pointsrightward, which has a component that is positively aligned to thereference magnetization direction 234, and will switch the free layer tothe right.

FIG. 3 illustrates an exemplary diagram of two different chopping statesof a bridge circuit for X-Y axis magnetic sensing according to variousembodiments of the invention. When a current pulse is applied to thecurrent line 410, a magnetic field pulse with a magnetization direction232 is generated on the first ferromagnetic layer. Depending on thepolarity of the applied current pulse, the generated magnetic fieldswitches the free layer direction 232 to have a component that ispositively or negatively aligned to the reference magnetizationdirection 234 of the second ferromagnetic layer.

FIG. 3 (A) shows a generally positively aligned magnetization direction232 in the first ferromagnetic layer 212 and FIG. 3 (B) shows agenerally negatively aligned magnetization direction 232 in the firstferromagnetic layer 212. In one embodiment, the cross angle between thegenerated magnetization direction 232 and the reference magnetizationdirection 234 is 45 degree under positive alignment or 225 degrees undernegative alignment. The above cross angles are described for exemplarypurposes of clarity and understanding. The cross angle may be any degreebetween 0 and ±90 under positive alignment or any degree between 180 and270 under negative alignment. Here the diagram is simplified so thateach sense element may indicate an array of sense elements.

FIG. 4 illustrates an exemplary diagram of two different chopping statesof a bridge circuit for Z axis magnetic sensing according to variousembodiments of the invention. For Z-axis magnetic sensing, each TMRelement integrates at least one flux guide 218. The flux guide 218 is ahigh aspect ratio vertical bar made from a high permeability materialwith ends terminating in close proximity to opposed edges of themagnetic sense element, in each respective bridge leg. In oneembodiment, the flux guide may be displaced above and/or below the first(free) ferromagnetic layer 212. The flux guide captures magnetic fluxfrom an applied field oriented in the Z direction, and bends the fieldlines to have a horizontal component near the ends of the flux guides.FIG. 4 (A) shows a +90 degree cross angle between the generatedmagnetization direction 232 and the reference magnetization direction234 and FIG. 4 (B) shows a −90 degree cross angle.

FIG. 5 is an exemplary control bipolar current signal 600 according tovarious embodiments of the invention. The reset/stabilization fieldsource 500 is configured to generate and apply a desired controlcurrent, such as the exemplary control bipolar current signal 600 asshown in FIG. 4, to the current line 410. The bipolar current signal 600is a cyclic signal comprising a positive portion 610 (+I_stab) and anegative portion 620 (−I_stab). In one embodiment, the bipolar currentsignal 600 further comprises a positive reset pulse 611 (+I_reset),which is placed at the beginning of the positive portion 610, functionedto switch the magnetization direction in a first direction. In oneembodiment, the bipolar current signal 600 further comprises a negativereset pulse 621 (−I_reset), which is placed at the beginning of thenegative portion 620, functioned to switch the magnetization directionin an opposite direction to the first direction.

The positive/negative reset pulses 611/621 may be very fast, being 10 to1M or more times smaller than the positive/negative portions, dependingon output rate, power, and noise requirements. The positive/negativeportions 610/620 may take any values including zero in certaincircumstances. In one embodiment, the positive portion 610 is equal tothe negative portion 620. In another embodiment, the positive portion610 is different from the negative portion 620.

In one embodiment, bipolar current signal 600 is symmetric with thepositive portion 610 and the negative portion 620 each occupying 50% ofevery cycle of the current signal 600. In one embodiment, bipolarcurrent signal 600 is asymmetric with the positive portion 610 and thenegative portion 620 occupying different percentage of every cycle ofthe current signal 600. In yet another embodiment, there may be a single−I reset pulse, and then a long series of +I reset pulses, where theinitial measurement sampled added to each subsequent measurementfollowing each +I reset pulse. For 1/f noise minimization, the bipolarcurrent signal 600 is applied at a rate of at least two times thedesired output data rate of the bridge output signal 260 of the magneticfield sensor 100. The 10 milliseconds cycle of the bipolar currentsignal 600 in FIG. 5 is for exemplary purpose and it is not intended tolimit the invention to the precise form disclosed. For implementation ofan offset subtraction and asymmetric cycle with an initial measurementphase maybe implemented.

FIG. 6 shows an exemplary bridge output signal 260 in response to theapplied bipolar current signal 600 in FIG. 4 according to variousembodiments of the invention. The bridge output signal 260 has a highoutput 261 and a low output 262 in response to the positive portion 610(+I_stab) and the negative portion 620 (−I_stab). The bridge outputsignal 260 responds asymmetrically to the symmetric bipolar currentsignal 600. In one embodiment, the high output 261 and the low output262 are both positive. In yet another embodiment, the high output 261 ispositive and the low output 262 is negative. For the applied field inthis exemplary case, the positive output 261 has a higher absolutevoltage value than the negative output 262. As a result, the outputsignal 260 has an overall average field dependent value, although thebipolar current signal 600 is symmetric and has a zero average value. Intypical circuit implementation the two values would be sampled and heldand added together to produce the output signal. As long as the bipolarcurrent signal 600 is applied at a rate of at least two times thedesired output data rate of the bridge output signal 260 of the magneticfield sensor 100, the implementation of bipolar current signal 600 hasno interference with the normal operating output of the magnetic fieldsensor 100.

FIG. 7 shows a diagram of validation measurement results over anextended period of time according to various embodiments of theinvention. The diagram includes many samples of individual signals 261to produce the positive signal train 271 (V_(pos)), includes manysamples of negative signals 262 to produce the negative signal train 272(V_(neg)), and a total signal 273 (V_(tot)). In FIG. 6, the Y-axis isoffset with a subtraction of a constant DC voltage to make all theresults fit on single clean plot. Each point of the positive signal 271and the negative signal 272 is taken after a delay after each resetpulse to eliminate ringing at transitions. Positive and negative signalsclearly invert at low frequency. Consequently, the total signal 273identifies a much reduced 1/f noise contribution with the positivesignal 271 and the negative signal 272 cancelling out each other.

FIG. 8 shows a multiple muxed (also called multiplexing) H bridgecircuit 800 for sequential reset and control logic according to variousembodiments of the invention. The muxed H bridge circuit 800 forms aplurality of single H bridge circuits for the current line 410 of eachTMR element 210. The muxed H bridge circuit 800 comprises a plurality oftransistors 810 to control current flow path in the current line 410.For example, p1 and n1 transistors may be closed, thus the current incurrent line 410 may follow a first reset path 801, then p2 and n2, etc.to allow similar reset path for the rest of the current linessequentially. Later a second reset path 802 in the opposite direction isestablished by controlling the open/close of desired transistorscombination, in this case p2 and n0. In one embodiment, the current pathof each current line 410 is controlled separately (sequentially). Inanother embodiment, the current paths of all current lines 410 arecontrolled together for a single unified current path for all thecurrent lines. With proper controlling logic of the mixed H bridgecircuit, the magnetic field sensor would be able to execute a firstmeasurement after a first reset pulse train, storage the firstmeasurement values, execute a second measurement after a second resetpulse train, storage the second measurement values, summation of bothmeasurement values to generate a total output. The mixed H bridgecircuit shown in FIG. 8 can be easily expanded or contracted toaccommodate various numbers of current line and transistors. Themultiple mixed H bridge circuit 800 is only for an exemplary purpose.Various modifications of the mixed H bridge circuit may be possiblewithin the scope and equivalence of the application.

FIG. 9 shows an exemplary read chain according to various embodiments ofthe invention. The read chain 900 comprises a plurality of componentsincluding magnetic bipolar reset chop 910, magnetic sensor 920,electrical chop 930, Low noise amplifier (LNA) 940, analog to digital(A/D) converter 950, electrical dechop 960, magnetic dechop 970, etc.The electrical chop 930, Low noise amplifier (LNA) 940, A/D converter950 and electrical dechop 960 form an electrical part 902 of the readchain, which may also include components for axes Muxing, downsampling,averaging, etc. In one embodiment, the magnetic bipolar reset chop 910operates in 1˜50,000 Hz frequency range, the electrical dechop 960operates in 40 kHz frequency, but is generally twice a magnetic chopfrequency, and the magnetic dechop 970 is matched to the magneticbipolar reset chop 910 frequency. In another embodiment, the abovecomponents may also work in other operating frequencies or order,depending on applications. The chop and de-chop pair may form a sampleand hold circuit that allows signal summing in each of the state,producing sensor output 273 as in FIG. 7.

One skilled in the art will recognize that various implementations maybe realized within the described architecture, all of which fall withinthe scope of the invention. For example, various parameters may beadjusted for the maximum elimination of the 1/f noise in TMR magneticfield sensors. Some of these parameters include reset pulse width andamplitude, bipolar current signal period, space ratio of the bipolarcurrent signal, etc.

The foregoing description of the invention has been described forpurposes of clarity and understanding. It is not intended to limit theinvention to the precise form disclosed. Various modifications may bepossible within the scope and equivalence of the application.

1-20. (canceled)
 21. A magnetic sensor, comprising: a first plurality ofmagnetoresistive sensor elements coupled together as a first circuit;and a second circuit adjacent to the first circuit and configured toreceive a bipolar current signal from a bipolar current source coupledto the second circuit, wherein a portion of the second circuit isadjacent to each magnetoresistive sensor element of the first pluralityof magnetoresistive sensor elements.
 22. The magnetic sensor of claim21, wherein each magnetoresistive sensor element comprises: a firstferromagnetic layer; a second ferromagnetic layer; and an intermediatelayer between the first and second ferromagnetic layers.
 23. Themagnetic sensor of claim 22, wherein the intermediate layer includes aninsulating material.
 24. The magnetic sensor of claim 21, wherein thebipolar current signal is configured to generate a magnetic fieldadjacent to each magnetoresistive sensor element of the first pluralityof magnetoresistance sensor elements.
 25. The magnetic sensor of claim21, wherein each magnetoresistive sensor element is one of a TunnelingMagnetoresistance (TMR) element, a Giant Magnetoresistance (GMR)element, or an Anisotropic Magnetoresistance (AMR) element.
 26. Themagnetic sensor of claim 21, wherein the first circuit is one of abridge circuit or a half bridge circuit.
 27. The magnetic sensor ofclaim 21, further comprising: a flux guide positioned adjacent to atleast one magnetoresistive sensor element of the first plurality ofmagnetoresistive sensor elements, wherein the flux guide is positionedabove or below the respective magnetoresistive sensor element.
 28. Themagnetic sensor of claim 21, further comprising: a second plurality ofmagnetoresistive sensor elements coupled together as a third circuit; afourth circuit configured to receive a bipolar current signal, wherein aportion of the fourth circuit is adjacent to each magnetoresistivesensor element of the second plurality of magnetoresistive sensorelements; a third plurality of magnetoresistive sensor elements coupledtogether as a fifth circuit; and a sixth circuit configured to receive abipolar current signal, wherein a portion of the sixth circuit isadjacent to each magnetoresistive sensor element of the third pluralityof magnetoresistive sensor elements.
 29. The magnetic sensor of claim28, further comprising: a flux guide positioned adjacent to at least onemagnetoresistive sensor element of the first plurality ofmagnetoresistive sensor elements.
 30. The magnetic sensor of claim 21,wherein the bipolar current signal is a cyclic signal comprising apositive portion and a negative portion, wherein the first circuit hasan output comprising a high output and a low output in response to thepositive portion and the negative portion, respectively, of the bipolarcurrent signal.
 31. A magnetic sensor, comprising: a plurality ofmagnetoresistive sensor elements coupled together as a first circuit,wherein each magnetoresistive sensor element comprises: a firstferromagnetic layer; a second ferromagnetic layer; and an intermediatelayer between the first and second ferromagnetic layers; and a secondcircuit adjacent to the first circuit and coupled to a bipolar currentsource, wherein a portion of the second circuit is adjacent to eachmagnetoresistive sensor element of the plurality of magnetoresistivesensor elements, and, in response to receiving a bipolar current signal,the second circuit is configured to generate a magnetic field adjacentto each magnetoresistive sensor element of the plurality ofmagnetoresistive sensor elements.
 32. The magnetic sensor of claim 31,wherein the generated magnetic field is configured to cancel at least aportion of 1/f noise generated from at least one magnetoresistive sensorelement of the plurality of magnetoresistive sensor elements.
 33. Themagnetic sensor of claim 31, wherein the intermediate layer includes aninsulating material.
 34. The magnetic sensor of claim 31, wherein atleast one magnetoresistive sensor element of the plurality ofmagnetoresistive sensor elements is a Tunneling Magnetoresistance (TMR)element.
 35. The magnetic sensor of claim 31, wherein the first circuitis one of a bridge circuit or a half bridge circuit.
 36. The magneticsensor of claim 31, wherein the first ferromagnetic layer of eachmagnetoresistive sensor element has a magnetization direction free torotate in a magnetic field, and the second ferromagnetic layer of eachmagnetoresistive sensor element has a fixed magnetization direction. 37.A magnetic sensor, comprising: a first plurality of magnetoresistivesensor elements coupled together as a first bridge circuit; a firstcircuit adjacent to the first bridge circuit and configured to receive abipolar current signal from a bipolar current source coupled to thefirst circuit, wherein a portion of the first circuit is adjacent toeach magnetoresistive sensor element of the first plurality ofmagnetoresistive sensor elements; a second plurality of magnetoresistivesensor elements coupled together as a second bridge circuit; and asecond circuit adjacent to the second bridge circuit and configured toreceive a bipolar current signal from a bipolar current source coupledto the second circuit, wherein a portion of the second circuit isadjacent to each magnetoresistive sensor element of the second pluralityof magnetoresistive sensor elements.
 38. The magnetic sensor of claim37, further comprising: a flux guide positioned adjacent to at least onemagnetoresistive sensor element of the first plurality ofmagnetoresistive sensor elements.
 39. The magnetic sensor of claim 37,further comprising: a third plurality of magnetoresistive sensorelements coupled together as a third bridge circuit; and a third circuitadjacent to the third bridge circuit and configured to receive a bipolarcurrent signal from a bipolar current source coupled to the thirdcircuit, wherein a portion of the third circuit is adjacent to eachmagnetoresistive sensor element of the third plurality ofmagnetoresistive sensor elements.
 40. The magnetic sensor of claim 37,wherein, in response to receiving the bipolar current signal, the firstcircuit is configured to generate a magnetic field adjacent to eachmagnetoresistive sensor element of the first plurality ofmagnetoresistive sensor elements.